Frequency stabilization system



April 12, 1938.

J. w. SMITH Er A1.

FREQUENCY STABILIZATION SYSTEM Filed oct. 17, 193e 4 Sheets-Sheet 1 HAIL/2ER A TTORNEY April 12, 1938.

' J. W. SMITH El' FRQUENCY STABILIZATION SYSTEM Filed. oct. 17. 1956 4 snets-shee't 2 nece/VE TnANsy/r /lv VENTO/as AG. N. THAYER- A Tiro/Mgr Aprily 12, 193s.

v 1. w. SMITH Er AL FREQUENCY STABILIZATIO .SYSTEM 4 Sheets-Sheet 5 Filed oct, 17, 1956 v J W SMITH /NvE/vTo/Ps M THAYER ATTORNEK April 12, 1938. .1.w. SMITH ET AL FREQUENCY STABILIZATION SYSTEM Filed Oct. 17, 1936 4 Sheets-Sh'eeb- 4 IN VE N TORS zei Jn:

- n'llam- /Th' YER G.N. er v ATTORNEY Patented Apr. 12, 1938 UNITED srii'iffiisy sume MQUENGY BTAB'ILIZAIQN SYSTEM Jenn w. animera-raton.' 'cnam u.A

Theron Glen aus. Telephon Laboratories. York, N. Y., corporation of New York N. I., to Bell Ineerporatedr 'Newv snpuesann'omm 1v. im. anni s. nous 'L s casina.v (Cl. lll-13 This invention relates to high frequency com munication systems and more particularly to frequency stabilization means therefor.

The principal obiect of the invention is to efg fect the stabilization of the carrier frequency -in very high .frequency communication systems. another object is to stabilize the mean frequency of the carrier wave in frequency modulation systems, and a third lobject isto eliminate or reduce unwanted frequency modulations in amplitude modulation systems. A further object is the elimination of distortion producing frequency .variations in frequency modulated systems.

In signaling systems designed to operate at 1l very high frequencies, for example, at frequencies corresponding toiradio waves of the order of one meter or less in length. modulation of the carrier wave may be-eifected by varying either its ampli tude or its frequency in proportion to the strength m of the modulating signal. In the case of amplitude modulation it is desirable Athat tho-carrier frequency bestabilized at a constant preassigned frequency and in the case of frequency modulation it is desirable that the mean frequency of the 2s .carrier be held constant.

4 When frequency modulation is used', the carrier wave lgenerator is preferably an oscillator of a type such that the frequency generated vcan be varied readily through a wide range under the w influence of an applied signal voltage. Fixed frequency oscillators, such as piezoelectric crystal controlled oscillators are not suited for this purpose and other available types are subject to' slow variations which. at very high frequencies, may amount to avery large number -of cycles. By the'present invention these slow drifts of a frequency modulated oscillator are substantially prevented while at the same time the desired frequency variations in accordance withthe modi-4 40 ulatingsignalare retained.

In amplitude modulation systemsoperating at the very high frequencies indicated above, relatively iarge power outputs are obtained most conveniently -by means of an oscillamon generator couples to the radiating antenna and oscillating directly at the carrier frequency. Crystal controlled osclllatorsare not welladapted to auch high `frequencies becausev of the minute dii-irgen-` u sions of the crystals required. Furthermore, oscillation generators of the available types for operation at these frequencies are generally subject to large simultaneous frequency modulations when amplitude modulation is attempted.. The' 55 -controlsystem of the invention substantially removes the unwanted frequency modulation 'with- I out affecting the amplitude modulation.

These results are accomplished in accordance v with the invention by a feedback system which picks up part of the radiated wave, analyses the 5 frequency variation thereof, converts the ,fre- .quency variations into corresponding amplitude variations of a control current, and applies the control current variations to the oscillator in which the radiated wave is generated in such 1o manner andinsuchphase asto oppose the frequency change.v

In the application of the invention to a fre quency modulation system. the varying control j current is fed back to thefrequency modulating le. device ci'.7 in eifect. to the moduiatingsignsl in. put circuit. The control current preferably re produces all of the signal variations togetherwith A,

' any other frequency deviations that may occur.

for example, slow changes due to instability and w other causes. The feedback is then of a stabilizing character and the system is analogous to the stabilized feemck amplicr mcribed by H. B. Black in an article entitled Stabiiiued Feedback Amplifiers, ml! System Technical Journal, Jan 25 uary 1934. ,By the use of a strong feedback or control applied in reverse phase to the signal input, undesired frequency deviations are substantiuy "im, DEM.

As stated above. preferably the whole of the gg detected signal and the accompanying undesired variations are fed back to exercise the stabilizing control. However, it is possible and may, in some cases, be desirable to feed hack only the slow variations for the purpose of preventing` slow 35 drifts of vthe mean frequency. This may he accomplished by a slight modification of the inven- Y tion.

when the invention is applied to amplitude modulation systems, the feedback or control is 40 not impressed on the signal input circuit, but on .an auxiliary frequency control means through which it exerts a quick acting compensating control of the' frequency.

Since the stabiiizins system of the invention 45 also acts as a detector for frequency modulated waves. it may be operated as a receiver vin conjunction with the transmitter. therebyproviding s two-way transmitting and receiving equipment in vwhich frequency stabilization of the transmitt'er is accomplished with a minimum of ap- Other features of the invention relate to limiting the range of operation of the frequency stabilisation equipment and indicating the operative 66 condition; toV switching arrangements whereby in Fig. y6 constitute a more detailed schematic representation of such a station; and

Fig. 5 shows the response curves of filters used in the system.

The system shown schematically in Fig. 1 comprises, essentially, a high frequency transmitter arranged for operation either with frequency modulation or with amplitude modulation and provided with feedback circuits for the stabilization of the mean radiated frequency. In addition to this, the system includes switching and circuit arrangements whereby part of the feedback circuit may be utilized for the detection and reception of incdming signals and a guard circuit for limiting the operation of the feedback control. The transmitter portion of the system and its stabilizing circuits will be described rst.

The carrier wave generator comprises a triod'e tube' i to the grid and plate of Vwhich are connected Lecher wires 2 and 2 forming an oscillation circuit. Preferably the oscillation generator is of the negative grid type, in which case, even at extremely .high frequencies, the frequency of 'the oscillations is determined mainly by the reactances' of the oscillation circuit. Plate current is supplied to the oscillator tube from battery l and a suitable negative bias is applied to the grid f by battery 4.- Alternatively the negative grid bias may be furnished by the fall of potential due to the plate current in choke coils 5 and l' inserted in the iament leads. Condenser O connected across the ends `of the Lecher wires forms a short circuit for the high frequency currents and high frequency 'chokes 'i4 and 1 serve further` to isolate the oscillation circuit `frolnthe current supply circuits. An antenna consisting of Adjustment or control of the carrier frequency is effected by a variable condenser comprising vilxed plates i5 and Il connected respectively to -the plate and grid of the oscillator tube and movable plate Il symmetricallydisposed with respect to the fixed plates. Plate Il is mounted on the moving element of a polarized electromagnetic :ging device I4.' the details oithe condenser its driving means v.being shown more fully in Fig. 2. The-motion of the' plate I1 isrestrained by a spring Il whichmakes the displacement substantially proportional to the ourrent delivered to thedevlce.

Microphone I0 'is connected through a signal amplifier Il and a two-way switch. l! 'eitherfto the-terminals of device I4 or to a transformer. i3, the secondary of which is included in the oscillator plate circuit. When switch i2 is -moved to the right the microphone is connected to the actuating winding of device Il. Amplified speech currents from Vlmicrophone l0 `will then produce synchronous displacements of condenser plate I1' proportional to the current intensity, thereby changing the natural Iperiod of the oscillator tuned circuit and effecting a variation of the carrier frequency substantially proportional to the signal. Whenl the microphone is connected to transformer I3, by operating switch I2 to the left, the amplified speech voltage is superimposed on the steady plate voltage of the oscillator thereby producing amplitude modulation of the carrier wave. 4

The feedback circuit includes an antenna consisting of a receiving doublet I8, i9 so placed as to receive waves radiated from antenna 8, 8'.

A transmission line 20 couples doublet i9, i9' tov generated oscillations being multiplied to a sultable high value by means of a harmonic generator 24.

' Amplification of the reduced frequency currents to a suitable degree is obtained in awide band 'amplifier 25 which provides substantially uniform gain over a frequency range wide enough vto accommodate all of the frequency variations likely to be encountered. The intermediate frequency amplifier preferably consists of a large'y number of stages, as shown in greater detail in Fig. 3, toproduce a strong feedback effect.

The portion of the feedback path following the intermediate frequency ampliile'rincludes la detector-converter circuit wherein is developed a control voltage corresponding in' magnitude and sign to the variations of the frequency ofthe carrier oscillations from a preassigned fixedv value.

the outputl terminals of amplifier !I, one path including buffer amplifier 2l.' frequency selective filter 21, rectifier 28, and output resistance 20,'

and the `other path including respectively corresponding elements 33, Il, 3l, and 3l. The elements in the two paths are similar with the exception of lthe illters 21 and. Il. v These are designed to produce attenuations 'which are equal at vthe lassigned value of the mean carrier frequency, but which vary in opposite senses with variations of .tha frequency therefrom. The

. filters may be high-pass and low-pass types respectively so proportioned that their attenuation characteristics cross in theranges 'of increasing attenuation adjacent the cut-ot frequencies. However, for operation at high frequencies, it will generally be more convenient to use simple resonant circuits tuned respectively to frequencies above and below the desired reference fre- 'fiers 2l and, the outputl voltages of filters 21 and will vary in amplitude in opposite ,senses as the oscillation frequency varies and will be equal at the frequencyedetermined by the intersection offthefllter characteristics. Rectification 40 The feedback circuit divides into two vpaths at Y ananas l of these voltagesby rectifiers 28 and I! will therefore produce variable low frequency voltages in y resistances 29 and II corresponding to the frequency variations of .the carrier. By connecting the resistances diiferentially'to a common cutput circuit. a differential voltage kis obtained therein which is directly proportional to the frequency vdeviation from the assigned carrier amplifiers 26 and-Il. Volume limitera of known typesmay also be used. for example, the tubes of amplifiers fl and Il may be operated at a point above plate current saturation so that'the outputs are held substantially constant.

.The differential control voltage derived from the detector-converter F circuit is applied to a push-pull direct current amplifier, l2, through an equalizer or filter network Il. vThe output terminals of the direct current amplier are connected to the actuating winding of control device I4, 4the connection being so poledy as to bring about the proper frequency compensating action.

Aitself due to instability or other causes.

The purpose of'equalizer network ll. is to insure that the control currents are maintained in the proper phase relation at all frequencies in the operating range. Usually this networkl may be omitted since the phase variation in the feedback loop is generally negligibly small.

'The stabilizing action of the .feedback control may now be described. n

Consider, first, the case -of frequency modulation, vfor which the input signal or currents -areimpressed directly upon the frequency control or modulating device Il. Variations of the radiated frequency corresponding to the sig-- nal currents are produced bythe action of device i4. These variable frequency oscillations are .picked up by antenna I9, i0', together with any other variations that may arise' in the e feedback circuit the detector-con' verterl reproduces the signal currents in'.I amplified form to- `gether with other current components corresponding to the unwanted frequencylvariations. All of these currents are fed back into the signal input circuit from the output of ampliiier 3|, lf,

in such phase as to oppose the original signal currents therein.

When the gain in the feedback circuit is large so :that a strong feedback is obtained, `the net result is a large reduction of the effective signal input voltage as compared with the voltage de livered by the signal ampliner Il. The gain in this ampliner must therefore be made large enough so that the net resultant inputv is lsufficient to produce the desired range of frequency modulation. We have found that with a strong reversed phase feedback, those .frequency variations which havetheir origin in the voscillator itself, and which therefore do` not correspond to the signal input, are reduced in magnitude in substantially the same proportion as the effective signal input voltage is reduced -by the feedback. `By making the 'feedback very large. the undesired frequency 'variations may be therefore substantiallyeeiiminatedr Both slow drifts of the oscillator frequency .due to. temperatureor v oltage changes and rapid shifts due-to other types.

of instability are eliminated. A frequency-modulated wave is thus obtained from which frequency that relay Il .is operated and the variations representing signal distortion are eliminated and which is stabilized about a fixed mean frequency. The elimination of the undesired frequency drifts is eected in a manner analogous to the elimination of non-linear distortion in the stabilized feedback amplifiers described in the lar-v modulatiomwhether electrical orv mechanical, and

of the type of oscillator. l

When the system' is operated with amplitude 'modulatiom the signal voltage is impressed on the plate of the oscillator vacuum tube through transformer il. The feedback circuit in this case does not reproduce the amplitude' variations of the carrier, because of the amplitude control of amplifiers 28 and 3ft, but does produce control currents corresponding to frequency variations of thecarrier. These will, in general, include variations due to oscillator instability and also vunwanted frequency modulations produced by and corresponding to the modulating signal voltage.' The control currents are fed back to control device i4, as in the case of frequency modulation,- and act thereon to produce a compensatingadjustinent of the oscillator frequency. Since there is no' degenerative feedback to\tl1e signal input circuit a less powerful signal input maybe used.

. For both of the methods of operation described above, it is desirable that the whole of the feedback loop be capable of transmitting freely all of the signal'I frequencies, for example the voice' feedback and the several advantageous operating characteristics mentioned above are not obtained. In order that the operation of the stabilizing With l feedback control may be confined tothe frequency range between the resonances of thefilters 21 and Il a guard circuit is provided winch interrupts the plate current supply to ampllner Si, 32, when the frequency approaches these limits.

Ihe guard circuit branches from the output of intermediate frequency amplifier 25 and includes amplifier Il, band-pass filter 41, detector load resistance B0, direct current ampliner ii and relay Il, the back contact of which controls the plate current supply to tubes 8l and 3 2. The pass-band of filter 41 lies between the resonance frequencies of filters 21, and 34. as indicated by curve u of Fig. 5. When the oscillations lie.

vwithin this range, relatively large currents areA passed to rectifier 4l and a correspondingly largenegative voltage is developed across resistance Il. This negative voltage is impressed on the gridof amplifier Il, reducing the plate current and causing relay B0 to release and close the ain-l pliiier plate circuit. When the oscillation frequency lies outside the pass-band of niter 41 substantially no current is passed. with the result ampiier is cathode temperature and the plate voltage of the oscillator tube are being established, the oscillation frequency varies through a very wide frequency range'. If the-stabilizing control were permitted to operate before the frequency has reached a value within the operating range of the converter lters, its action would be reversed and the operation would tend to drive the oscillation frequency towards either a zero or an innitely large value. If the filters could be. made to have continuously rising characteristics, this could not happen.y Such filters, however. are difficult and costly to construct for operation at radio fre'- quencies. Wide frequency variations may also occur when the oscillator plate voltage is applied after the cathode hasbeen fully heated. In this case the frequency reaches approximately its normal value very rapidly.

The guard circuit also serves to provide auto- I matic volume control for the ampliers of the feedback path. For this purpose a resistance 48 shunted by acondenser of low capacity is inserted in series with rectifier Il and the output of filter 41. The rectified current develops a negative voltage in this resistor which is transmitted by conductor 52 to the several amplifiers 25, 28, 33 and demodulator 2| and is-applied to the grids of the vacuum tubes thereof. The capacity in shunt toresistance 49 should be large enough to act as a by-pass for the carrier frequency currents, but not large enough to reduce the signal frequency variations of the rectified voltage. TheY rapidity of the control may be adjusted by,suitable resistance-capacity nlters which may be included l within the amplifiers.V For demodulator 2| and station operating on vthe same frequency as the carrier oscillator. For this purpose a push-pull signal amplifier 40, 4| is connected'y by switches 42 and Il to the output of'thedetector-converter, the amplier output being coupled to a telephone receiver M. While receiving, the transmitting oscillator may be disabled by interrupting itsV plate Acurrent supply by means of transmit-re- Iceive switch j". With this connection the system is in condition for the reception of frequency modulated waves.- For the reception of amplitude modulated waves, switch l2 -is arranged to connect amplifier Il iii-.parallel with the input of guard circuit amplifier 5| and switch I3 is operated to disable amplifier 4l.

With two systems of the above type operating` on the same frequency, a two-way communication channel can be established. Switching from the transmitting condition to the receiving condition is effected by the operation of the single switch 45. Changing from frequencymodulation to amplitude modulation is effected ation of switches I2, l2, and". i t

In atypical example, the carrier oscillator was designed to operate at a frequency of 300 megacycles per second. The heterodyne frequency furnished by harmonic generator 24 was ,295 megacycles, the intermediate-frequency of the feedbackcurrents being ve million cycles per second. Filters 21 and Il 'were designed to have -their characteristics cross at this frequency and to have their resonances separated by about 400,000 cycles. The degreeof stability obtained was thus substantially that of the crystal oscillator 22, since any variations of the control point of filters 21 and 34 could affect only the relatively low intermediate frequency and could affect that by the operonly to a very small extent. ,l

Details of the circuits of the complete-system of Fig. l are shown in Figs. 2, 3, and 4, which nt together in themanner indicated by Fig. 6. Fig. 2 shows the circuits of the carrier wave oscillator and constructional features of the tuning control device I4. Fig. 3 shows the circuits of the heterodyne generator comprising oscillator 22 and harmonic producer 2l and of demodulator 2l and intermediate frequency amplifier 2l. Fig. 4 shows the remaining portion of the feedback circuit and the guard circuit.

In its preferred form, as shown in Fig. 2, the tuning control ldevice I4 -is of the electromagnetic moving coil type. The movable condenser plate i1 is attached rigidly to the supporting frame l5 of the moving coil, its plane coinciding with the coil axis. Fixed plates Il and Il are disposed symmetrically with respect to plate Il and parallel thereto with about 50 per cent .overlap for 'the normal position of the coil. VMotion of the coil along 'its axis thus varies thecapacity4 between plates l5 and I0 substantially proportionally to the displacement of thecoil. Springs I8 act as the coil restraint and also serve to hold it centrally in the magnet air-gap. Resonance of the moving system may be prevented by applying mechanical damping thereto, for example, by covering the surfaces of springs 'Il 'with dissipative material, such as felt, or with a thin coating of lead. The damping should not be great enough to prevent yresponse of the device at any frequency in the signal range. Alternatively, electrical damping may -be provided bya suitable ,resistance 54 connected acrossy the moving coil terminals. A eld winding Il .is provided for polarizing the magnetic circuit.

The power for all of the vacuum tubes and 'for magnet winding I8 is supplied by full-wave rectiner s1, sa. Fig. s, and an associated smoathingm- :sol

ter, connected to' a 60 cycle alternating current l source 59. A separate winding 6| is provided onthe power input'transformer for energizing the vacuum tube cathodes.' An additional smoothing l filter is included in the oscillator platel supply circuit as'a further precaution against unwanted `frequency modulations. ,The cathodes of all of the vacuumtubesfexcept the guard circuit input amplifier lt-rare connected to ground at a point intermediate the positive and vnegative potentials 'of thepower supply. v 'Harmonic generator M comprises three frequency multiplying-.stages each using screen-grid pentode tubes with indirectly heated cathodes. The output circuitof oscillator 22 is so arranged that a harmonic of the generated frequency is supplied to the input of the harmonic generator, thereby securing additional frequency multiplication. Demodulator 2i also uses `a screen-grid pentode tube with indirectly heated cathode, the posing said varying strength currents upon the Iincoming oscillations being impressed on the conoscillationsv from said signal source to countertrol grid and the heterodyne oscillations on the actthe frequency modulation of the carrier wave. suppressor grid between the screen and the anode. 2. In a high frequency communication system,

l Intermediate frequency amplifier 25 comprises a carrier wave generator, means for modulating 5 live pentode stages coupled together and to the the frequency of said generator in accordance demodulator by double tuned circuit broad-band with signal currents, a source of signal currents, a filters. signal input circuit coupling said source and said The output of amplifier 25 is fed into buffer modulating means, a feedback path coupling the amplifiers 26 and 33 in the plate circuits of which output lof said generator and said signal input 10 are connected the converter filters 21 and 34. circuit, said, feedbackvpath including means for These filters consist of simple anti-resonant cirreproducing from the carrier wave currents havcuits tuned to different frequencies and proporing strength variations corresponding to the tioned to provide response characteristics inter-i frequency modulations thereof, and means forsecting in the manner shown by curves 52 and impressing said varying strength currentsv upon 15 53. The rectiflers shown, at 28 and 35 inl Fig. 1 said input circuit, the transmission characterisare constituted by the space paths between auxiltic of said lfeedback path and its connection to lary' anodes 62 and 63 and the cathodes of tubes said input circuit being such that the fed back 40 and 4I, which are of the diode-triode type. currents are in substantially opposite phase re- Theswitches 42 and I3 of Fig. 1 are combined lation to the signal currents from said source and 20 in a two-way double throw switch 61. The' outare approximately equal thereto in magnitude.

put of ampliflers 40 and 4l goes to transformer 3. A system in accordance with claim 2 in 66 and thence through an additional low frewhich the said modulating device is responsive to quency amplifier 69 to telephone. receiver 44. signal frequency currents and frequencies down Guard circuit amplier 46 has its control grid ex'- 'to zero, and in which the feedback path is-respon- 25 cited in parallel with amplifiers 26 and 33 by thesive to frequency variations at corresponding output of amplifier 26. Bandi-pass lter I1 comrates.

prises a pair of tuned circuits suitably coupled to 4. In a' high frequency communication system, provide the desired pass-band. Rectifier I8 is a carrier wave oscillator comprising a tuned circonstituted by the space paths between auxiliary cuit which determines the frequency of the gen- 30 anodes 65 and 66 of amplifier tube 5i which is a erated waves, a variable tuning element in said double diode-triode. Electrodes 66 and 66' are tuned clrcuit,'means determining a substantialconnected together through a condenser., the outly fixed frequency, means for automatically adput of filter 41 being'connected to anode 66. Rejusting said tuning element in response to de l 3154 sistance A50 from which theI grid of tube 5I is Vexpartures of said oscillator from said fixed fre- 35 cited is connected in shunt between diode 65 and quency, and means simultaneously controlling the tube cathode. A small high frequencyl choke said tuning element in accordance with signals. coil is included in series with this resistance to 5. In a highfrequency communication system, suppress the carrier frequency currents.l Resista carrier wave oscillator comprising a tuned cir-l 40 ance 49 in which the automatic volume control cuit which determines the frequency ofthe gen- 40 potential is developed is included in series beerated waves, a variable tuning element in said tween the output of filter I1 and the cathode of tuned circuit. means for determining a substanthe tube. Connections from taps in this resisttially xed frequency, means for varyingv said y ance are led to the grids of the several controlled tuning element in accordancewith signals, means 45 amplifiers and provide volume control voltages detecting departures of the oscillator from the 45 of suitable magnitudes therefor. Lead 10 supfixed frequency, said detecting means being opplies the control voltage to the grids of amplifiers erative over a range of frequenciesincluding the 26 and 33 through filter 1l, the time constant of signals, and means 4for automatically-adjusting which is made sufllciently small so thatV signal the tuning element in response to frequency defrequency variations are transmitted. Control Partllres 0f Signal and l10n-Signal Origin. y A 60 bias is supplied to the tubes of amplier 26 and 6. In a high frequency communication system, demodulator 2| through lead 12 and filter 13, the a carrier wave oscillator comprising a tuned cirtime constant of which is large enough to percuit which determines the frequency of the genmit only slow variations to be transmitted. lli erated waves, a' variable tuning element in said 66 'steady initial grid bias for all of these tubes is tuned circuit. means determining e substantially 55 provided by the fall of potentialin resistance 16 fixed frequency, means foil varying said tuning which is connected in series with resistance 15 element 'in accordance with signals, means for between the negative terminal of the power supdetecting departures of the oscillator from the ply rectifier and the cathodes. The indicator xed frequency. scid detecting means beine re- 39, meter or lamp, is included inan auxiliary cirsponsive to slow departures of frequency and to so cuit connected to the front contact of .relay 60 departures at signal rates. and means for autoliei plate curmatlcally adjusting said tuning element in rerather than directly in the amp rent circuit as m Fig, 1, sponse to said departures of both kinds.

What is claimed is: Y l 1. In a high frequency communication system, a carrier wave oscillator compris y a carrier' wave generator, a, source of signal oscuit which determines the frequency of the genr impressing oscillations from erated waves, a variable reactance element in said cillations, means fo y said source upon said generator whereby the fretuned circuiti means determining ai Substantially ting means for automatiquency of the generated carrier wave is modu- Xed frequency. slow acl lated in accordance with the signal oscillations, cally adusting said variable clement in response 7o means for detecting the frequency modulations to slow departures of the said oscillator frequency of the carrier wave in the output of said genern from said fixed frequency and separate means forv ator, means for deriving from said detected frecontrolling said variable element in. accordance quency modulations currents having correspondwith signals; 16 ing strength variations, and means for superimj 8. In a combined '1. In a high frequency communication system,

transmitting and receivin l5 ing a tuned cir- 65 u.

with frequency uctuations, and means to disable the`carrier oscillator during reception of waves from another station.

9. In afrequency stabilized oscillator system comprising an oscillator generator, frequency correcting means therefor, means determining a substantially fixed frequency, and means for automatically adjusting said correcting means in l response to departures of the generated frequency from said fixed frequency, an auxiliary transmission path coupled to the output of said generator and responsive to impressed oscillations in a limited range of frequencies centered about said fixed frequency, and current responsive means in the output of said path for disabling said corrective means when the frequency of the generated oscillations lies outside said limited range.

JOHN W. SMITH. GORDON N. THAYER. 

